A multi-carrier communication system, such as a Discrete Multiple-Tone (DMT) system in the various types of Digital Subscriber Line (e.g., ADSL and VDSL) systems, carries information from a transmitter to a receiver over a number of sub-channels or tones. DMT is a two line-modulation technique used for VDSL. A DMT communication system divides signals into multiple separate channels or tones. Each tone acts as a separate communication channel to carry information between a local transmitter-receiver device and a remote transmitter-receiver device. Each tone is a group of one or more frequencies defined by a center frequency and a set bandwidth.
DMT communication systems use a modulation method in which the available bandwidth of a communication channel, such as twisted-pair copper media, is divided into these numerous tones. The term communication channel is understood to refer generally to a physical transmission medium, including copper, optical fiber, and so forth, as well as other transmission mediums, including radio frequency (RF) and other physical or non-physical communication signal paths.
There are various types of interference and noise sources in a multi-carrier communication system, such as the DMT system. Interference and noise may corrupt the data-bearing signal on a tone as the signal travels through the communication channel and is decoded at the receiver. The transmitted data-bearing signal may further be decoded erroneously by the receiver because of this signal corruption. The number of data bits or the amount of information that a tone carries may vary from tone to tone and depends on the relative power of the data-bearing signal compared to the power of the corrupting signal on that particular tone. Thus, a measure of the quality of signal carried by a tone is the ratio of the received signal strength (power) over the noise strength in the frequency range of operation, or the Signal to Noise Ratio (SNR). High SNR results in high signal quality being carried by a tone. Another measure of signal quality is bit error rate (BER) for a given tone.
In order to account for potential interference on the transmission line and to guarantee a reliable communication between the transmitter and receiver, each tone is typically designed to carry a limited number of data bits per unit time based on the tone's SNR using a bit-loading algorithm. The number of bits that a specific tone may carry decreases as the relative strength of the corrupting signal increases, that is when the SNR is low or the BER is high. Thus, the SNR of a tone may be used to determine how much data should be transmitted by the tone.
It is often assumed that the corrupting signal is an additive random source with Gaussian distribution and white spectrum. With this assumption, the number of data bits that each tone can carry relates directly to the SNR. However, this assumption may not be true in many practical cases and there are various sources of interference that do not have a white, Gaussian distribution. Impulse noise is one such noise source. Bit-loading algorithms, which are methods to determine the number of bits per tone, are usually designed based on the assumption of additive, white, Gaussian noise. With such algorithms, the effects of impulse noise are underestimated resulting in an excessive rate of error.
Further, channel estimation procedures can be designed to optimize performance in the presence of stationary impairments such as additive, white, Gaussian noise, but are often poor at estimating non-stationary or cyclo-stationary impairments, such as impulse noise. Consequently, DSL modem training procedures are typically well suited to optimizing performance in the presence of additive, white, Gaussian noise, but leave the modem receivers relatively defenseless to impulse noise.
Impulse Noise can be a difficult impairment for DSL modems. Impulse noise with duration of tens of microseconds can cause errors in all the used sub-channels at the receiver. Further, because a DMT system programs the impulse noise protection for entire DMT frames, not just for the length of impulse found in the network, latency is introduced in the system.
Typically, impulse noise is mitigated by a combination of two methods: forward error correction (FEC) with interleaving and reducing data rate for increased noise margin. One mitigation strategy compatible with current ADSL2 and VDSL1 procedures is to operate the receiver with sufficient noise margin (including Reed-Solomon coding and interleaving) to maintain the error rate within acceptable limits. Unfortunately, this means that 90-98% of frames are typically running with excess margin to ensure the integrity of data in 2-10% of the frames directly impacted by impulse noise. Further, this method generally does not work for low-latency applications, and it requires a very large interleaver depth, which increases system memory requirements.